The present invention relates to a method of unidirectionally aligning whiskers during tape casting and a sintered silicon nitride ceramic laminate consisting of layers containing unidirectionally oriented silicon nitride whiskers fabricated by using the method.
Advanced ceramic materials have been attracting a lot of attention as candidate materials for certain parts of advanced machinery due to their excellent mechanical and/or chemical properties. Thanks to intensive R&D activities, there are many advanced ceramics parts serving in the advanced and more efficient machinery today.
However, there still remain barriers to more widespread use of the advanced ceramics, one of which is their brittleness. Many efforts have been made to make ceramic materials of higher fracture toughness. Whisker reinforced ceramic composites have been developed to improve the fracture toughness of the ceramic materials. Whiskers of high aspect ratio are effective for resisting the crack propagation as summarized by Becher in the Journal of the American Ceramic Society, 74, 255-69 (1991).
Since excellent properties along a specific direction was expected and observed from a composite materials containing aligned whiskers, many research workers have made efforts to control the orientation of the whiskers of the composite. Muscat et al. reported on the microstructure of sialon containing 15% of .beta. silicon nitride whiskers which were aligned in the extrusion direction in the Journal of the American Ceramic Society, 75, 2713-18 (1992). Goto and Tsuge reported on the mechanical properties of unidirectionally oriented SiC whisker reinforced silicon nitride fabricated by extrusion and hot pressing in the Journal of the American Ceramic Society, 76, 1420-24 (1993). They noticed not only higher fracture toughness in the direction normal to the whiskers but also higher flexural strength for the specimen containing aligned whiskers. Wu and Messing reported on the fabrication of mullite matrix composite containing aligned SiC whiskers by tape casting method. They prepared the composite containing 30 volume % of SiC whiskers. They noticed that the whiskers became better aligned as the casting speed increased, but it should be very high for an appreciable alignment.
Silicon nitride ceramic materials are often called "self-reinforced ceramic materials" due to their microstructural development during sintering at high temperature. The equiaxial fine grains of the starting silicon nitride powder dissolve into the liquid and reprecipitate as needle-like grains, some of which grow large enough to act as reinforcements. The microstructural change is more noticeable if the starting powder is of .alpha. phase. Since Wittmer et al. reported on the development of .beta. silicon nitride for self-reinforced composites by using the "seeding" method in Ceramic Engineering & Science Proceedings, 13, 907-917 (1992), many researchers have employed the method for controlling the microstructures of their silicon nitride specimens. Hirao et al. prepared their own seed crystals of .beta. silicon nitride average aspect ratio of which was about 4.5 as they reported in the Journal of the American Ceramic Society, 77, 1857-1892 (1994). They also prepared the silicon nitride laminates containing the seed crystals and noticed that the large elongated grains in the sintered specimen were lying randomly in each layer as reported in the Journal of the American Ceramic Society, 78, 1687-90 (1995).